Winter is a good time to get a leg up on pests and diseases through the use of dormant sprays. Dormant sprays are applied to deciduous trees and shrubs after the leaves have fallen and the plants are dormant.
About Dormant Sprays
Dormant sprays kill overwintering insects and fungal diseases that can threaten the plant next spring.
Because the pests have limited hiding places in winter, organic dormant spraying can reduce the need for more harmful treatments and sprays during the growing season.
Dormant sprays are most often used on:
Other deciduous trees or shrubs that struggled with infestation or disease during the previous growing season.
Types of Dormant Sprays
There are three main types of dormant sprays. In general, all three types are considered organic, but be sure to check the label before using, since some dormant sprays are mixed with non-organic pesticides.
Lime Sulfur: (calcium polysulphide) Lime sulfur usually comes in liquid form and is an effective fungicide against diseases such as powdery mildew, black spot, apple scab, rust, and other fungal diseases. The sulfur smell can also be a deterrent to munching deer and other critters.
Copper: Also called Bordeaux mixture, copper comes either in liquid or powder form, sometimes formulated as copper sulfate. While copper is more hazardous (and should be kept out of groundwater), it’s popular because of its effectiveness against bacterial infection as well as fungal diseases.
Dormant (horticultural) Oil: Dormant oil sprays work by coating and smothering overwintering insects and eggs. Dormant oil can be used to control pests such as aphids, scale, mites, twig borers, white flies, and leaf rollers. Dormant oils are generally nontoxic except to insects. Since they work by smothering insects, the critters can’t build up resistance. The most common dormant oils, such as Volck oil and Bonide, are petroleum based. Others, including Oil-Away Supreme Insecticidal Oil, are plant based. Dormant oil can also be used as a base mixed with other insecticides or fungicides, to make them stick and work better.
Dormant Spraying Tips
Follow Instructions: Dormant sprays are usually sold as concentrates that can be sprayed with a garden sprayer or hose attachment. Be sure to mix and apply according to package instructions – more isn’t necessarily better, and just because they’re organic doesn’t mean they’re harmless.
Coverage: In order for the sprays to be effective, they have to cover every nook and cranny, including the undersides of branches, usually until the plant is dripping.
Temperature: Check the label, but most sprays work best in temperatures at least over 40° F and ideally over 50° F.
Timing: Dormant sprays are typically applied in December and again in mid-February, although some require just one spraying in January or February. Spray on a mild day with no rain, wind, or freezing temperatures forecast for the next 24 hours, to give the spray a chance to spread and dry. For best results, spray after pruning in late winter, just before the spring buds begin to swell on the plant.
Apply Only as Needed: As with all sprays, use only when needed on plants that have suffered from pests or disease. Widespread preventative spraying will kill beneficial insects as well.
Use Only on Dormant Plants: Unless otherwise labeled, dormant sprays can harm leaves and flowers on growing plants, so use only when plants are dormant.
Spraying Safety: Even though they’re organic, dormant sprays can be irritating to skin and eyes. Wear long sleeves and gloves, and use eye protection and a mask when applying dormant sprays.